Mesoporous carbons are used as adsorbents or catalysts supports and can be used in spherical, granular of thin film form. Existing production methods use gas phase and chemical activation routes to produce mesoporous carbons but, Activated carbon, as conventionally produced, is normally microporous (<2 nm pore diameter—IUPAC definition) with little or no pore volume in the mesopore (2-50 nm) and macropore (>50 nm) range. For some critical adsorption processes such as evaporative emission control, and when used as a catalyst support, particularly in liquid phase applications, this is a major drawback.
Conventional activated carbons can be made mesoporous through severe activation but this seriously degrades their mechanical properties and the materials are generally then only available as fine powders. U.S. Pat. No. 4,677,086 discloses the use of chemical activation to produce mesoporous carbons without such severe mechanical degradation and which also can be produced as extrudates. These are however still produced as powders and must then be bound to produce, for instance, extrudate for use in fixed bed gas phase processes. In most cases the binders that can be used are polymeric or ceramic which then restricts the conditions under which the carbons can be used.
Chemical activation can also be used to directly produce mesoporous carbons by pelleting or extruding a plasticised acidic lignin base char and then directly carbonising and activating the mixture as disclosed in U.S. Pat. No. 5,324,703. The production route also leads to a low macroporosity, which can have disadvantages in catalytic and liquid phase processes. The route also has the disadvantage of requiring compounds such as phosphoric acid and zinc chloride as the activating agents, which can cause severe environmental problems and have a major impact on the materials of construction of the process plant.
Phenolic resins can be carbonised to form mesoporous carbons.
As to the technical background concerning phenolic resins, sulphonated phenolic resins were first used as ion exchange resins in the 1930's (Adams et al, J Soc Chem Ind., 54, (1935) 1-GT) and relatively stable cation and anion exchange resins were used extensively for the softening and demineralisation of water. Other phenolic based resins include the weak base anion exchange resins that have been primarily used in food processing applications (Cristal M J, Chem and Ind, 814, (1983) November 7) and chelation resins which can be produced to give remarkable selectivity for the adsorption of metal ions such as cesium (U.S. Pat. No. 4,423,159, 1983 and U.S. Pat. No. 5,441,991, 1995). The ion exchange powders, can be produced by either bulk curing of the resin followed by milling (e.g. WO91/09891) to produce a low porosity powder or by reversed phase condensation (Unitaka Ltd U.S. Pat. No. 4,576,969 1986). One of the limitations of these materials was limited internal porosity and they were rapidly replaced by the highly porous sulphonated styrene divinyl benzene copolymer based ion exchange resins when these became available. However, although the phenolic based resins have largely disappeared, specific applications do still exist in food related industries based on their underlying performance characteristics. RU-A-2015996 (Plastmassy) discloses the sulphuric acid catalysed production of ion exchange resins from phenol, formaldehyde and hexamethylene tetramine, but neither discloses nor suggests carbonisation of the resulting resin.
U.S. Pat. No. 6,024,899 (Peng) discloses making mesoporous (>50 vol. % pores in the 2-50 nm size range) carbon by combining a carbon precursor and a pore former which dissolves molecularly into the carbon precursor. The precursor is preferably a phenolic resin owing to its low viscosity, high carbon yield and high degree of cross-linking on curing, a resole resin being preferred. The pore former is preferably a thermoplastic material and has a decomposition or volatilisation temperature above a curing temperature and below a carbonisation temperature of the carbon precursor. The carbon precursor is cured and carbonised, pores being formed by loss of the mass of pore former from the resulting solid solution of pore former in the carbon precursor. In an example a phenolic resole in methanol is mixed with polyvinyl butyral (PVB) also in methanol as pore former so that the amount of PVB is about 15% of that of the phenolic resin. The resulting solution is dip-coated onto a ceramic substrate which is dried, cured at 100° C. and carbonised at 750° C. to give a carbonised product of surface area 200 m2/g and 95% pore volume in the mesopore range. A second example uses polyethylene glycol as pore former and a third example uses coal tar pitch as the pore former. However, the only quoted pore size is for the third example, where the pores are said to be of diameter predominantly 10 nm.
CN-A-1247212 (Shanxi) discloses the production of mesoporous carbon from a phenolic resin using a complex of iron, cobalt or nickel e.g. ferrocene, cobalt acetylacetonate or nickelocene which is effective to give rise to mesopores catalytically after carbonisation when the carbonised resin is activated. Production of mesoporous carbon in bead form is described. In a procedure described in a first example, a novolak (linear phenolic aldehyde resin), hexamethylene tetramine and anhydrous methanol are heated until all the ingredients have dissolved, after which the methanol is vacuum evaporated and the resulting uncured mixture of resin and crosslinker is crushed to give particles of size 450-1250 μm. The particles are alleged to be cured to form beads by dispersion in water containing surfactant and heating to 125° C. after which the beads were heated for a period of about 6 hours to 700° C. to effect carbonisation with a dwell of 30 minutes at 700° C. However, there is no mesoporosity introduced at the resin stage, nor is there mesoporosity introduced at the carbonisation stage. There are no escape paths for gaseous resin decomposition products, so that physical strength, attrition resistance and freedom of dust cannot be expected. Furthermore, steam activation causes the organometallic pore-formers of Shanxi to form mesopores by catalysis of reaction between the carbon and the steam by clusters of metal atoms, but the resulting pores are small (diameters 7.4-9.4 nm, see Table 2 in that specification).
Furthermore, the applicants have repeated the Shanxi experimental work, and although autoclave curing to a hard resinous material was achieved with a strong smell of ammonia/amines that are secondary products of cross-linking reaction of novolak and hexamethylene tetramine, formation of beads could not be obtained either at a relatively low stirrer speed or at the maximum possible stirrer speed. The cured product took the form of several irregular large lumps of hard resin with a colour of ripe peas and some small fibre shaped particles and powder. There was no evidence of mesoporosity on carbonisation and, in contrast to the results reported by Shanxi, only a very low level of mesoporosity on activation.
An alternative route is to carbonise sulphonated styrene—divinylbenzene co-polymers as disclosed in U.S. Pat. No. 4,040,990 and U.S. Pat. No. 4,839,331. These produce carbons directly by pyrolysis with meso/microporosity without recourse to further activation. The materials therefore have good mechanical properties. They are, however, limited to relatively small particle sizes, fixed by the polymer production route, and have a limited range of mesopore structures. They are also very expensive reflecting the high cost of the precursor polymer, the low carbon yields and environmental problems associated with processing polymers containing large amounts of sulphur. The resultant carbons are also contaminated with sulphur, which restricts their use as catalysts supports.
A further route has also been disclosed in U.S. Pat. No. 5,977,016 whereby sulphonated styrene—divinylbenzene co-polymer particles can be formed into pellets in the presence of large volume of concentrated sulphuric acid and then carbonised to give structured materials with both meso- and macroporosity. The route is however complex and expensive with significant environmental problems.
A further route is disclosed in U.S. Pat. No. 4,263,268 where a mesoporous silica with the desired macroshape (i.e. spheres) is impregnated with a carbon forming polymer, such as phenolic or polyfurfuryl resin and then dissolving the silica template in an alkali. This again is a highly expensive route and is only capable of producing the carbon material in a limited range of shapes and forms.